Method to program a memory cell comprising a carbon nanotube fabric element and a steering element

ABSTRACT

A method of programming a carbon nanotube memory cell is provided, wherein the memory cell comprises a first conductor, a steering element, a carbon nanotube fabric, and a second conductor, wherein the steering element and the carbon nanotube fabric are arranged electrically in series between the first conductor and the second conductor, and wherein the entire carbon nanotube memory cell is formed above a substrate, the carbon nanotube fabric having a first resistivity, the method including applying a first electrical set pulse between the first conductor and the second conductor, wherein, after application of the first electrical set pulse, the carbon nanotube fabric has a second resistivity, the second resistivity less than the first resistivity. Other aspects are also provided.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/692,144, filed Mar. 27, 2007, now U.S. Pat. No. 7,667,999, entitled“Method To Program a Memory Cell Comprising a Carbon Nanotube Fabric anda Steering Element,” which is incorporated by reference herein in itsentirety for all purposes.

This application is related to Herner et al. U.S. patent applicationSer. No. 11/692,148, “Memory Cell Comprising a Carbon Nanotube FabricElement and a Steering Element,” Herner U.S. patent application Ser. No.11/692,151, “Method to Form Upward-Pointing P-I-N Diodes Having Largeand Uniform Current,” and Herner U.S. Pat. No. 7,586,773, “Large Arrayof Upward-Pointing P-I-N Diodes Having Large and Uniform Current,” allhereby incorporated by reference in their entirety.

BACKGROUND

Carbon nanotube memories are believed to operate by flexing ofindividual carbon nanotubes or carbon nanotube ribbons in an electricfield. This flexing mechanism requires space within which the carbonnanotubes can flex. In nanotechnologies, forming and maintaining such anempty space is extremely difficult.

It would be advantageous to form a memory cell using carbon nanotubeswhich is readily fabricated. It would further be advantageous to formsuch a memory cell in a highly dense, very large cross-point array.

SUMMARY

In a first aspect of the invention, a method of programming a carbonnanotube memory cell is provided, wherein the memory cell comprises afirst conductor, a steering element, a carbon nanotube fabric, and asecond conductor, wherein the steering element and the carbon nanotubefabric are arranged electrically in series between the first conductorand the second conductor, and wherein the entire carbon nanotube memorycell is formed above a substrate, the carbon nanotube fabric having afirst resistivity, the method including applying a first electrical setpulse between the first conductor and the second conductor, wherein,after application of the first electrical set pulse, the carbon nanotubefabric has a second resistivity, the second resistivity less than thefirst resistivity.

In a second aspect of the invention, a method is provided, the methodincluding: (a) providing a memory cell comprising a first conductor, asecond conductor, a steering element and a carbon nanotube materialarranged electrically in series between the first conductor and thesecond conductor; and (b) applying a first electrical pulse between thefirst conductor and the second conductor to change a resistivity of thecarbon nanotube material.

In a third aspect of the invention, a method is provided for forming amemory cell, the method including forming a first conductor, forming asteering element above the first conductor, forming a carbon nanotubematerial coupled in series with the steering element and forming asecond conductor above the carbon nanotube material.

In a fourth aspect of the invention, a method is provided for forming amonolithic three dimensional memory array, the method including: (a)forming a first memory level including a memory cell, wherein formingthe memory cell includes forming a first conductor, forming a steeringelement above the first conductor, forming a carbon nanotube materialcoupled in series with the steering element and forming a secondconductor above the carbon nanotube material; and (b) monolithicallyforming a second memory level above the first memory level.

Other features and aspects of the present invention will become morefully apparent from the following detailed description, the appendedclaims and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Features of the present invention can be more clearly understood fromthe following detailed description considered in conjunction with thefollowing drawings, in which the same reference numerals denote the sameelements throughout, and in which:

FIG. 1 is a perspective view of a memory cell formed according to apreferred embodiment of the present invention.

FIG. 2 is a perspective view of a portion of a first memory levelcomprising memory cells like those shown in FIG. 1.

FIGS. 3 a-3 c include various views showing a memory array formedaccording to an embodiment of the present invention.

FIG. 4 is a cross-sectional view of another embodiment of the presentinvention.

FIGS. 5 a-5 d are cross-sectional views illustrating stages in formationof two monolithically formed memory levels of a monolithic threedimensional memory array formed according to a preferred embodiment ofthe present invention.

DETAILED DESCRIPTION

A carbon nanotube is a hollow cylinder of carbon, typically a rolledsheet a single carbon atom thick. Carbon nanotubes typically have adiameter of about 1-2 nm and length hundreds or thousands of timesgreater.

Nonvolatile memories retain information even when power to the device isturned off. A nonvolatile memory cell using carbon nanotubes isdescribed in, for example, Segal et al. U.S. Pat. No. 6,643,165,“Electromechanical memory having cell selection circuitry constructedwith nanotube technology,” and in Jaiprakash et al. U.S. Pat. No.7,112,464, “Devices having vertically-disposed nanofabric articles andmethods of making the same.”

In both Segal et al. and Jaiprakash et al., a carbon nanotube element(either a single carbon nanotube or a carbon nanotube ribbon of multipletubes) is spatially separate from an electrode, the carbon nanotubeelement either horizontally oriented and suspended above an electrode,or vertically oriented and adjacent to a vertically oriented electrode.The memory cell operates by exposing the carbon nanotube element to anelectric charge, causing the carbon nanotube element to mechanicallyflex, making electrical contact with the electrode. These two electricalstates of the memory cell, with the carbon nanotube element either incontact or not in contact with the adjacent electrode, can be sensed,remain when power is removed to the device, and correspond to twodistinguishable data states of the memory cell.

As the mechanism relies on movement of the carbon nanotube element, astructure must be fabricated with a gap between the carbon nanotubeelement and the adjacent electrode to allow such movement. Fabricationof such a gap is difficult at very small dimensions, and will becomemore so as dimensions continue to shrink.

In the present invention, a nonvolatile memory cell is formed using acarbon nanotube fabric. The term carbon nanotube fabric will be usedherein to describe a contiguous plurality of carbon nanotubes with norequired orientation of the individual tubes, in contrast to a carbonnanotube ribbon, in which the nanotubes must be substantially parallel.In preferred embodiments, such a carbon nanotube fabric includes severalor many layers of carbon nanotubes at random orientations. Operation ofthe cell does not require creation of open space within which individualnanotubes can flex, and thus will be more robust and simpler tofabricate.

It is expected that carbon nanotube fabric will exhibit resistivityswitching behavior; i.e. the fabric will change its resistivity whensubjected to sufficient voltage or current. A switch from higherresistivity to lower resistivity will be referred to as a settransition, which is achieved by an electrical set pulse, while a resettransition, from lower resistivity to higher resistivity, is achieved byan electrical reset pulse. The terms set voltage, set current, resetvoltage, and reset current will also be used.

Summarizing, then, in one embodiment, the cell includes a steeringelement and a carbon nanotube fabric arranged electrically in seriesbetween a first conductor and a second conductor. The carbon nanotubefabric may be in a first state, having a first resistivity. Afterapplication of a first electrical set pulse between the first conductorand the second conductor, the carbon nanotube fabric has a secondresistivity, the second resistivity less than the first resistivity.Next, after application of a first electrical reset pulse across thesteering element and the carbon nanotube fabric, the carbon nanotubefabric has a third resistivity, the third resistivity greater than thesecond resistivity. The data state of the memory cell can be stored inany of these resistivity states. A read voltage is applied afterapplication of the first set pulse, or application of the first resetpulse, to sense the data state.

FIG. 1 shows an embodiment of the present invention. Carbon nanotubefabric 118 and diode 302 are disposed electrically in series betweenbottom conductor 200 and top conductor 400. Optional conductive barrierlayers 110 and 111 sandwich carbon nanotube fabric 118. In oneembodiment, when this memory cell is formed, carbon nanotube fabric 118is in a first resistivity state, for example a high-resistivity or resetstate. In the reset state, when a read voltage is applied between topconductor 400 and bottom conductor 200, little or no current flowsbetween the conductors. After application of a set pulse, theresistivity of carbon nanotube fabric 118 undergoes a set transition tothe set state, which is a low-resistivity state. With carbon nanotubefabric 118 in the set state, when the same read voltage is appliedbetween top conductor 400 and bottom conductor 200, significantly morecurrent flows between them. After application of a reset pulse, theresistivity of carbon nanotube fabric 118 undergoes a reset transition,returning to a high-resistivity reset state. When read voltage isapplied between top conductor 400 and bottom conductor 200,significantly less current flows between them. The different currentunder applied read voltage between the set state and the reset state canbe reliably sensed. These different states can respond to distinct datastates of a memory cell; for example one resistivity state maycorrespond to a data “0” while another corresponds to a data “1. ” In analternative embodiment, the initial state of carbon nanotube fabric 118may be a low-resistivity state. For simplicity, two data states will bedescribed. It will be understood by those skilled in the art, however,that three, four, or more reliably distinguishable resistivity statesmay be achieved in some embodiments.

FIG. 2 shows a plurality of bottom conductors 200 and top conductors400, with intervening pillars 300, the pillars 300 comprising diodes andcarbon nanotube fabric elements. In an alternative embodiment, the diodecould be replaced with some other non-ohmic device. In this way a firstlevel of memory cells can be formed; only a small portion of such amemory level is shown here. In preferred embodiments, additional memorylevels can be formed stacked above this first memory level, forming ahighly dense monolithic three dimensional memory array. The memory arrayis formed of deposited and grown layers above a substrate, for example amonocrystalline silicon substrate. Support circuitry is advantageouslyformed in the substrate below the memory array.

An alternative embodiment of the present invention uses a structuredescribed in Petti et al., U.S. patent application Ser. No. 11/143,269,“Rewriteable Memory Cell Comprising a Transistor andResistance-Switching Material in Series,” filed Jun. 2, 2005, assignedto the assignee of the present invention and hereby incorporated byreference. Petti et al. describe a memory cell having a layer of aresistivity-switching binary metal oxide or nitride formed in serieswith a MOS transistor. In embodiments of Petti et al., the MOStransistor is a thin-film transistor, having its channel layer formed indeposited polycrystalline semiconductor material rather than in amonocrystalline wafer substrate.

Turning to FIG. 3 a, in a preferred embodiment of Petti et al. aplurality of substantially parallel data lines 10 is formed.Semiconductor pillars 12 are formed, each above one of the data lines10. Each pillar 12 includes heavily doped regions 14 and 18 which serveas drain and source regions, and a lightly doped region 16 which servesas a channel region. A gate electrode 20 surrounds each pillar 12.

FIG. 3 b shows the cells of FIG. 3 a viewed from above. In a repeatingpattern, pitch is the distance between a feature and the next occurrenceof the same feature. For example, the pitch of pillars 12 is thedistance between the center of one pillar and the center of the adjacentpillar. In one direction pillars 12 have a first pitch P1, while inother direction, pillars 12 have a larger pitch P2; for example P2 maybe 1.5 times larger than P₁. (Feature size is the width of the smallestfeature or gap formed by photolithography in a device. Stated anotherway, pitch P₁ may be double the feature size, while pitch P2 is threetimes the feature size.) In the direction having the smaller pitch P₁,shown in FIG. 3 a, the gate electrodes 20 of adjacent memory cellsmerge, forming a single select line 22. In the direction having largerpitch P2, gate electrodes 20 of adjacent cells do not merge, andadjacent select lines 22 are isolated. FIG. 3 a shows the structure incross-section along line X-X′ of FIG. 3 b, while FIG. 3 c shows thestructure in cross-section along line Y-Y′ of FIG. 3 b.

Referring to FIGS. 3 a and 3 c, reference lines 24, preferablyperpendicular to data lines 10, are formed above the pillars 12, suchthat each pillar 12 is vertically disposed between one of the data lines10 and one of the reference lines 24. A resistance-switching memoryelement 26 is formed in each memory cell between source region 18 andreference line 24, for example. Alternatively, resistance-switchingmemory element 26 can be formed between drain region 14 and data line10. In preferred embodiments of the present invention,resistance-switching element 26 comprises a layer of carbon nanotubefabric. Note that in the embodiment of FIGS. 3 a-3 c, the carbonnanotube fabric is at the top of the pillar rather than below it.

FIG. 4 illustrates another embodiment of Petti et al. This embodimentsimilarly includes memory cells in a TFT array, each having a transistorand a reversible resistance-switching memory element in series, but hasa different structure. Substantially parallel rails 30 (shown in crosssection, extending out of the page) include a plurality of line sets 31,each line set 31 consisting of two data lines 32 and one reference line34, reference line 34 immediately adjacent to and between the two datalines 32. Above the rails 30 and preferably extending perpendicular tothem, are substantially parallel select lines 36. Select lines 36 arecoextensive with gate dielectric layer 38 and channel layer 40. Thememory level includes pillars 42, each pillar 42 vertically disposedbetween one of the channel layers 40 and one of the data lines 32 or oneof the reference lines 34. Transistors are formed comprising adjacentpillars along the same select line. Transistor 44 includes channelregion 51 between source region 50 and drain region 52. One pillar 42 aincludes resistance-switching element 46, while the other pillar 42 bdoes not. In this embodiment, adjacent transistors share a referenceline; for example transistor 48 shares a reference line 34 withtransistor 44. No transistor exists between adjacent data lines 32. In apreferred embodiment of the present invention, resistance-switchingelement 46 comprises a layer of carbon nanotube fabric.

In the embodiments of FIG. 1 and FIGS. 3 a-3 c and FIG. 4, the carbonnanotube fabric is paired with a diode or a transistor. A diode and atransistor share the characteristic of non-ohmic conduction. An ohmicconductor, like a wire, conducts current symmetrically, and currentincreases linearly with voltage according to Ohm's law. A device thatdoes not follow these rules exhibits non-ohmic conduction, and will bedescribed as a steering element. By pairing a steering element with acarbon nanotube fabric, memory cells can be formed in a largecross-point array. The steering element provides electrical isolationbetween adjacent cells such that a selected cell can be set, reset, orsensed without inadvertently setting or resetting cells sharing awordline or bitline with the selected cell.

Each of these embodiments includes a first conductor; a steeringelement; a carbon nanotube fabric; and a second conductor, wherein thesteering element and the carbon nanotube fabric are arrangedelectrically in series between the first conductor and the secondconductor, and wherein the entire memory cell is formed above asubstrate.

These embodiments are provided as examples; others can be envisioned andfall within the scope of the invention.

As described in Herner et al., U.S. patent application Ser. No.11/148,530, “Nonvolatile Memory Cell Operating by Increasing Order inPolycrystalline Semiconductor Material,” filed Jun. 8, 2005, herebyincorporated by reference, when deposited amorphous silicon iscrystallized in contact solely with materials with which it has a highlattice mismatch, such as silicon dioxide and titanium nitride, thepolycrystalline silicon or polysilicon forms with a high number ofcrystalline defects, causing it to be high-resistivity. Application of aprogramming pulse through this high-defect polysilicon apparently altersthe polysilicon, causing it to be lower-resistivity.

As described further in Herner et al. U.S. patent application Ser. No.10/955,549, “Nonvolatile Memory Cell Without a Dielectric AntifuseHaving High- and Low-Impedance States,” filed Sep. 29, 2004; in HernerU.S. Pat. No. 7,176,064, “Memory Cell Comprising a SemiconductorJunction Diode Crystallized Adjacent to a Silicide,” both herebyincorporated by reference, it has been found that when depositedamorphous silicon is crystallized in contact with a layer of anappropriate silicide, for example titanium silicide or cobalt silicide,the resulting crystallized silicon is much higher quality, with fewerdefects, and has much lower resistivity. The lattice spacing of titaniumsilicide or cobalt silicide is very close to that of silicon, and it isbelieved that when amorphous silicon is crystallized in contact with alayer of an appropriate silicide at a favorable orientation, thesilicide provides a template for crystal growth of silicon, minimizingformation of defects. Unlike the high-defect silicon crystallizedadjacent only to materials with which it has a high lattice mismatch,application of a large electrical pulse does not appreciably change theresistivity of this low-defect, low-resistivity silicon crystallized incontact with the silicide layer.

Referring to FIG. 1, in a preferred embodiment, diode 302 is preferablya junction diode. The term junction diode is used herein to refer to asemiconductor device with the property of conducting current more easilyin one direction than the other, having two terminal electrodes, andmade of semiconducting material which is p-type at one electrode andn-type at the other. Examples include p-n diodes, which have p-typesemiconductor material and n-type semiconductor material in contact, andp-i-n diodes, in which intrinsic (undoped) semiconductor material isinterposed between p-type semiconductor material and n-typesemiconductor material. In the embodiment of FIG. 1, diode 302 ispreferably formed of silicon and the bottom layer of top conductor 400is a silicide-forming metal such as titanium or cobalt. An anneal causesthe silicon of diode 302 to react with the silicide-forming metal,forming a layer of a silicide such as titanium silicide or cobaltsilicide, which provides a crystallization template for the silicon ofdiode 302, causing it to be formed of high-quality, low-resistivitysilicon. Thus a set or reset pulse applied between conductor 400 and 200serves only to switch the resistivity state of carbon nanotube fiber118, and not to change the resistivity of the silicon of diode 302. Thismakes set and reset transitions more controllable and predictable, andmay serve to reduce the amplitude of the pulse required. In otherembodiments, the silicon of diode 302 may be deposited amorphous and maybe crystallized adjacent only with materials with which it has a highlattice mismatch, and thus may formed of be high-defect,high-resistivity polysilicon.

This discussion has described a diode formed of silicon crystallized incontact with an appropriate silicide. Silicon and germanium are fullymiscible, and the lattice spacing of germanium is very close to that ofsilicon. It is expected that alloys of amorphous silicon-germaniumcrystallized in contact with an appropriate silicide-germanide (such astitanium silicide-germanide or cobalt silicide-germanide) will similarlycrystallize to form low-defect, low-resistivitypolysilicon-polygermanium.

The preferred diode in the present invention is a vertically orientedp-i-n diode, having a bottom heavily doped region of a firstconductivity type, a middle intrinsic or lightly doped region, and a topheavily doped silicon of a second conductivity type opposite the first.

A detailed example will be provided describing fabrication of two memorylevels formed above a substrate, the memory levels comprising memorycells having a diode and a carbon nanotube fabric element arranged inseries between a bottom conductor and a top conductor. Details fromHerner U.S. patent application Ser. No. 11/560,283, “P-I-N DiodeCrystallized Adjacent to a Silicide in Series with a DielectricAntifuse,” filed Nov. 15, 2006, hereby incorporated by reference, mayprove useful in fabrication of this memory level. To avoid obscuring theinvention, not all details from this or other incorporated documentswill be included, but it will be understood that no teaching of theseapplications and patents is intended to be excluded. For completeness,many details, including materials, steps, and conditions, will beprovided, but it will be understood by those skilled in the art thatmany of these details can be changed, augmented or omitted while theresults fall within the scope of the invention.

EXAMPLE

Turning to FIG. 5 a, formation of the memory begins with a substrate100. This substrate 100 can be any semiconducting substrate known in theart, such as monocrystalline silicon, IV-IV compounds likesilicon-germanium or silicon-germaniumcarbon, III-V compounds, II-VIIcompounds, epitaxial layers over such substrates, or any othersemiconducting material. The substrate may include integrated circuitsfabricated therein.

An insulating layer 102 is formed over substrate 100. The insulatinglayer 102 can be silicon oxide, silicon nitride, Si—C—O—H film, or anyother suitable insulating material.

The first conductors 200 are formed over the substrate 100 and insulator102. An adhesion layer 104 may be included between the insulating layer102 and the conducting layer 106 to help conducting layer 106 adhere toinsulating layer 102. If the overlying conducting layer 106 is tungsten,titanium nitride is preferred as adhesion layer 104. Conducting layer106 can comprise any conducting material known in the art, such astungsten, or other materials, including tantalum, titanium, or alloysthereof.

Once all the layers that will form the conductor rails have beendeposited, the layers will be patterned and etched using any suitablemasking and etching process to form substantially parallel,substantially coplanar conductors 200, shown in FIG. 5 a incross-section. Conductors 200 extend out of the page. In one embodiment,photoresist is deposited, patterned by photolithography and the layersetched, and then the photoresist removed using standard processtechniques.

Next a dielectric material 108 is deposited over and between conductorrails 200. Dielectric material 108 can be any known electricallyinsulating material, such as silicon oxide, silicon nitride, or siliconoxynitride. In a preferred embodiment, silicon dioxide deposited by ahigh-density plasma method is used as dielectric material 108.

Finally, excess dielectric material 108 on top of conductor rails 200 isremoved, exposing the tops of conductor rails 200 separated bydielectric material 108, and leaving a substantially planar surface. Theresulting structure is shown in FIG. 5 a. This removal of dielectricoverfill to form the planar surface can be performed by any processknown in the art, such as chemical mechanical planarization (CMP) oretchback. In an alternative embodiment, conductors 200 could be formedby a Damascene method instead.

Turning to FIG. 5 b, next optional conductive layer 110 is deposited.Layer 110 is a conductive material, for example titanium nitride,tantalum nitride, or tungsten. This layer may be any appropriatethickness, for example about 50 to about 200 angstroms, preferably about100 angstroms. In some embodiments barrier layer 110 may be omitted.

Next a thin layer 118 of carbon nanotube fabric is formed using anyconventional method. (For simplicity substrate 100 is omitted from FIG.5 b and succeeding figures; its presence will be assumed.) In someembodiments this layer can be formed by spin casting or spray coating asolution including carbon nanotubes; such solutions are commerciallyavailable. Carbon nanotube fabric layer 118 is preferably between about2 nm and about 500 nm thick, most preferably between about 4 and about40 nm thick.

Conductive layer 111 is deposited on layer 118. It can be anyappropriate conductive material, for example titanium nitride, with anyappropriate thickness, for example about 50 to about 200 angstroms,preferably about 100 angstroms. In some embodiments conductive layer 111may be omitted.

Conductive layers 110 and 111, which are immediately below andimmediately above carbon nanotube fabric 118, respectively, and inpermanent contact with it, will serve as electrodes, and may aid inresistivity switching of carbon nanotube fabric 118. The layer to bedeposited next is a semiconductor material, such as silicon, typicallydeposited by a low-pressure chemical vapor deposition (LPCVD) process.Silicon deposited by LPCVD has excellent step coverage, and, ifdeposited directly on carbon nanotube fabric 118, may tend to infiltratebetween the individual carbon nanotubes, changing the composition andbehavior of the fabric. Conductive layer 111, formed of a material withpoorer step coverage, helps to prevent such infiltration.

Next semiconductor material that will be patterned into pillars isdeposited. The semiconductor material can be silicon, germanium, asilicon-germanium alloy, or other suitable semiconductors, orsemiconductor alloys. For simplicity, this description will refer to thesemiconductor material as silicon, but it will be understood that theskilled practitioner may select any of these other suitable materialsinstead.

Bottom heavily doped region 112 can be formed by any deposition anddoping method known in the art. The silicon can be deposited and thendoped, but is preferably doped in situ by flowing a donor gas providinga p-type dopant atoms, for example boron, during deposition of thesilicon. In preferred embodiments, the donor gas is BC1₃, and p-typeregion 112 is preferably doped to a concentration of about 1×10²¹atoms/cm³. Heavily doped region 112 is preferably between about 100 andabout 800 angstroms thick, most preferably about 200 angstroms thick.

Intrinsic or lightly doped region 114 can be formed next by any methodknown in the art. Region 114 is preferably silicon and has a thicknessbetween about 1200 and about 4000 angstroms, preferably about 3000angstroms. The silicon of heavily doped region 112 and intrinsic region114 is preferably amorphous as deposited.

Semiconductor regions 114 and 112 just deposited, along with underlyingconductive layer 111, carbon nanotube fabric 118, and conductive layer110, will be patterned and etched to form pillars 300. Pillars 300should have about the same pitch and about the same width as conductors200 below, such that each pillar 300 is formed on top of a conductor200. Some misalignment can be tolerated.

Pillars 300 can be formed using any suitable masking and etchingprocess. For example, photoresist can be deposited, patterned usingstandard photolithography techniques, and etched, then the photoresistremoved. Alternatively, a hard mask of some other material, for examplesilicon dioxide, can be formed on top of the semiconductor layer stack,with bottom antireflective coating (BARC) on top, then patterned andetched. Similarly, dielectric antireflective coating (DARC) can be usedas a hard mask.

The photolithography techniques described in Chen U.S. patentapplication Ser. No. 10/728,436, “Photomask Features with InteriorNonprinting Window Using Alternating Phase Shifting,” filed Dec. 5,2003; or Chen U.S. patent application Ser. No. 10/815,312, “PhotomaskFeatures with Chromeless Nonprinting Phase Shifting Window,” filed Apr.1, 2004, both owned by the assignee of the present invention and herebyincorporated by reference, can advantageously be used to perform anyphotolithography step used in formation of a memory array according tothe present invention.

The diameter of the pillars 300 can be as desired, for example betweenabout 22 nm and about 130 nm, preferably between about 32 nm and about80 nm, for example about 45 nm. Gaps between pillars 300 are preferablyabout the same as the diameter of the pillars. Note that when a verysmall feature is patterned as a pillar, the photolithography processtends to round corners, such that the cross-section of the pillar tendsto be circular, regardless of the actual shape of the correspondingfeature in the photomask.

Dielectric material 108 is deposited over and between the semiconductorpillars 300, filling the gaps between them. Dielectric material 108 canbe any known electrically insulating material, such as silicon oxide,silicon nitride, or silicon oxynitride. In a preferred embodiment,silicon dioxide is used as the insulating material.

Next the dielectric material on top of pillars 300 is removed, exposingthe tops of pillars 300 separated by dielectric material 108, andleaving a substantially planar surface. This removal of dielectricoverfill can be performed by any process known in the art, such as CMPor etchback. After CMP or etchback, ion implantation is performed,forming heavily doped n-type top regions 116. The n-type dopant ispreferably a shallow implant of arsenic, with an implant energy of, forexample, 10 keV, and dose of about 3×10¹⁵/cm². This implant stepcompletes formation of diodes 302. The resulting structure is shown inFIG. 5 b. Fabrication of the p-i-n diode 302 is described in more detailin Herner, U.S. patent Ser. No. 11/692,151, “Method to FormUpward-Pointing P-I-N Diodes Having Large and Uniform Current,” filedMar. 27, 2007, on even date with U.S. patent application Ser. No.11/692,144 from which this application claims priority. Note that somethickness, for example about 300 to about 800 angstroms of silicon islost during CMP; thus the finished height of diode 302 may be betweenabout 800 and about 4000 angstroms, for example about 2500 angstroms fora diode having a feature size of about 45 nm.

Turning to FIG. 5 c, next a layer 120 of a silicide-forming metal, forexample titanium, cobalt, chromium, tantalum, platinum, niobium, orpalladium, is deposited. Layer 120 is preferably titanium or cobalt; iflayer 120 is titanium, its thickness is preferably between about 10 andabout 100 angstroms, most preferably about 20 angstroms. Layer 120 isfollowed by titanium nitride layer 404. Layer 404 is preferably betweenabout 20 and about 100 angstroms, most preferably about 80 angstroms.Next a layer 406 of a conductive material, for example tungsten, isdeposited; for example this layer may be about 1500 angstroms oftungsten formed by CVD. Layers 406, 404, and 120 are patterned andetched into rail-shaped top conductors 400, which preferably extend in adirection perpendicular to bottom conductors 200. The pitch andorientation of top conductors 400 is such that each conductor 400 isformed on top of and contacting a row of pillars 300. Some misalignmentcan be tolerated.

Next a dielectric material (not shown) is deposited over and betweenconductors 400. The dielectric material can be any known electricallyinsulating material, such as silicon oxide, silicon nitride, or siliconoxynitride. In a preferred embodiment, silicon oxide is used as thisdielectric material.

Referring to FIG. 5 c, note that layer 120 of a silicide-forming metalis in contact with the silicon of top heavily doped region 116. Duringsubsequent elevated temperature steps, the metal of layer 120 will reactwith some portion of the silicon of heavily doped region 116 to form asilicide layer (not shown), which is between the diode and top conductor400; alternatively this silicide layer can be considered to be part oftop conductor 400. This silicide layer forms at a temperature lower thanthe temperature required to crystallize silicon, and thus will formwhile regions 112, 114, and 116 are still largely amorphous. If asilicon-germanium alloy is used for top heavily doped region 116, asilicide-germanide layer may form, for example of cobaltsilicide-germanide or titanium silicide-germanide.

In the example just described, the diode 302 of FIG. 5 c comprises abottom heavily doped p-type region, a middle intrinsic region, and topheavily doped n-type region. In preferred embodiments, the next memorylevel to be monolithically formed above this one shares conductor 400with the first memory level just formed; i.e., the top conductor 400 ofthe first memory level serves as the bottom conductor of the secondmemory level. If conductors are shared in this way, then the diodes inthe second memory level preferably point the opposite direction,comprising a bottom heavily doped n-type region, a middle intrinsicregion, and a top heavily doped p-type region.

Turning to FIG. 5 d, next optional conductive layer 210, carbon nanotubefabric layer 218, and optional conductive layer 211 are formed,preferably of the same materials, the same thicknesses, and using thesame methods as layers 110, 118, and 111, respectively, of pillars 300in the first memory level.

Diodes are formed next. Bottom heavily doped region 212 can be formed byany deposition and doping method known in the art. The silicon can bedeposited and then doped, but is preferably doped in situ by flowing adonor gas providing n-type dopant atoms, for example phosphorus, duringdeposition of the silicon. Heavily doped region 212 is preferablybetween about 100 and about 800 angstroms thick, most preferably about100 to about 200 angstroms thick.

The next semiconductor region to be deposited is preferably undoped. Indeposited silicon, though, n-type dopants such as phosphorus exhibitstrong surfactant behavior, tending to migrate toward the surface as thesilicon is deposited. Deposition of silicon will continue with no dopantgas provided, but phosphorus atoms migrating upward, seeking thesurface, will unintentionally dope this region. As described in HernerU.S. patent application Ser. No. 11/298,331, “Deposited SemiconductorStructure to Minimize N-Type Dopant Diffusion and Method of Making,”filed Dec. 9, 2005, hereby incorporated by reference, the surfactantbehavior of phosphorus in deposited silicon is inhibited with theaddition of germanium. Preferably a layer of a silicon-germanium alloyincluding at least 10 at % germanium is deposited at this point, forexample about 200 angstroms of Si_(0.8)Ge_(0.2), which is depositedundoped, with no dopant gas providing phosphorus. This thin layer is notshown in FIG. 5 d.

Use of this thin silicon-germanium layer minimizes unwanted diffusion ofn-type dopant into the intrinsic region to be formed, maximizing itsthickness. A thicker intrinsic region minimizes leakage current acrossthe diode when the diode is under reverse bias, reducing power loss.This method allows the thickness of the intrinsic region to be increasedwithout increasing the overall height of the diode. As will be seen, thediodes will be patterned into pillars; increasing the height of thediode increases the aspect ratio of the etch step forming these pillarsand the step to fill gaps between them. Both etch and fill are moredifficult as aspect ratio increases.

Intrinsic region 214 can be formed next by any method known in the art.Region 214 is preferably silicon and preferably has a thickness betweenabout 1100 and about 3300 angstroms, preferably about 1700 angstroms.The silicon of heavily doped region 212 and intrinsic region 214 ispreferably amorphous as deposited.

Semiconductor regions 214 and 212 just deposited, along with underlyingconductive layer 211, carbon nanotube fabric 218, and conductive layer210, will be patterned and etched to form pillars 500. Pillars 500should have about the same pitch and about the same width as conductors400 below, such that each pillar 500 is formed on top of a conductor400. Some misalignment can be tolerated. Pillars 500 can be patternedand etched using the same techniques used to form pillars 300 of thefirst memory level.

Dielectric material 108 is deposited over and between the semiconductorpillars 500, filling the gaps between them. As in the first memorylevel, the dielectric material 108 on top of pillars 500 is removed,exposing the tops of pillars 500 separated by dielectric material 108,and leaving a substantially planar surface. After this planarizationstep, ion implantation is performed, forming heavily doped p-type topregions 216. The p-type dopant is preferably a shallow implant of boron,with an implant energy of, for example, 2 keV, and dose of about3×10¹⁵/cm². This implant step completes formation of diodes 502. Theresulting structure is shown in FIG. 5 d. Some thickness of silicon islost during the CMP step, so the completed diodes 502 have a heightcomparable to that of diodes 302.

Top conductors 600 are formed in the same manner and of the samematerials as conductors 400, which are shared between the first andsecond memory levels. A layer 220 of a silicide-forming metal isdeposited, followed by titanium nitride layer 604 and layer 606 of aconductive material, for example tungsten. Layers 606, 604, and 220 arepatterned and etched into rail-shaped top conductors 600, whichpreferably extend in a direction substantially perpendicular toconductors 400 and substantially parallel to conductors 200.

Preferably after all of the memory levels have been formed, a singlecrystallizing anneal is performed to crystallize the semiconductormaterial of diodes 302, 502, and those diodes formed on additionallevels, for example at 750 degrees C. for about 60 seconds, though eachmemory level can be annealed as it is formed. The resulting diodes willgenerally be polycrystalline. Since the semiconductor material of thesediodes is crystallized in contact with a silicide or silicide-germanidelayer with which it has a good lattice match, the semiconductor materialof diodes 302, 502, etc. will be low-defect and low-resistivity.

In the embodiment just described, conductors were shared between memorylevels; i.e. top conductor 400 of the first memory level serves as thebottom conductor of the second memory level. In other embodiments, aninterlevel dielectric (not shown) is formed above the first memory levelof FIG. 5 c, its surface planarized, and construction of a second memorylevel begins on this planarized interlevel dielectric, with no sharedconductors. In the example given, the diodes of the first memory levelwere downward-pointing, with p-type silicon on the bottom and n-type ontop, while the diodes of the second memory level were reversed, pointingupward with n-type silicon on the bottom and p-type on top. Inembodiments in which conductors are shared, diode types preferablyalternate, upward on one level and downward on the next. In embodimentsin which conductors are not shared, diodes may be all one type, eitherupward- or downward-pointing. The terms upward and downward refer to thedirection of current flow when the diode is under forward bias.

In the embodiment just described, referring to FIG. 5 d, in the firstmemory level carbon nanotube fabric 118 was disposed between diode 302and bottom conductor 200; and, in the second memory level, between diode502 and bottom conductor 400. In other embodiments, the carbon nanotubefabric element may be disposed between a vertically oriented diode and atop conductor.

In some embodiments, it may be preferred for the programming pulse to beapplied with the diode in reverse bias. This may have advantages inreducing or eliminating leakage across the unselected cells in thearray, as described in Kumar et al. U.S. patent application Ser. No.11/496,986, “Method For Using A Memory Cell Comprising SwitchableSemiconductor Memory Element With Trimmable Resistance,” filed Jul. 28,2006, owned by the assignee of the present invention and herebyincorporated by reference.

To summarize, what has been described is a first memory levelmonolithically formed above a substrate, the first memory levelcomprising: i) a plurality of first substantially parallel,substantially coplanar bottom conductors; ii) a plurality of steeringelements; iii) a plurality of first-level carbon nanotube fabricelements, and iv) a plurality of first substantially parallel,substantially coplanar top conductors; and v) a plurality of first-levelmemory cells, wherein each first-level memory cell comprises one of thesteering elements and one of the first-level carbon nanotube fabricelements arranged electrically in series between one of the first bottomconductors and one of first top conductors; and (b) a second memorylevel monolithically formed above the first memory level.

A monolithic three dimensional memory array is one in which multiplememory levels are formed above a single substrate, such as a wafer, withno intervening substrates. The layers forming one memory level aredeposited or grown directly over the layers of an existing level orlevels. In contrast, stacked memories have been constructed by formingmemory levels on separate substrates and adhering the memory levels atopeach other, as in Leedy U.S. Pat. No. 5,915,167, “Three dimensionalstructure memory.” The substrates may be thinned or removed from thememory levels before bonding, but as the memory levels are initiallyformed over separate substrates, such memories are not true monolithicthree dimensional memory arrays.

A monolithic three dimensional memory array formed above a substratecomprises at least a first memory level formed at a first height abovethe substrate and a second memory level formed at a second heightdifferent from the first height. Three, four, eight, or indeed anynumber of memory levels can be formed above the substrate in such amultilevel array.

An alternative method for forming a similar array in which conductorsare formed using Damascene construction is described in Radigan et al.U.S. patent application Ser. No. 11/444,936, “Conductive Hard Mask toProtect Patterned Features During Trench Etch,” filed May 31, 2006,assigned to the assignee of the present invention and herebyincorporated by reference. The methods of Radigan et al. may be usedinstead to form an array according to the present invention. In themethods of Radigan et al., a conductive hard mask is used to etch thediodes beneath them. In adapting this hardmask to the present invention,in preferred embodiments the bottom layer of the hard mask, which is incontact with the silicon of the diode, is preferably titanium, cobalt,or one of the other silicide-forming metals mentioned earlier. Duringanneal, then, a silicide forms, providing the silicide crystallizationtemplate mentioned earlier.

Detailed methods of fabrication have been described herein, but anyother methods that form the same structures can be used while theresults fall within the scope of the invention.

The foregoing detailed description has described only a few of the manyforms that this invention can take. For this reason, this detaileddescription is intended by way of illustration, and not by way oflimitation. It is only the following claims, including all equivalents,which are intended to define the scope of this invention.

1. A method for programming a carbon nanotube memory cell, wherein thememory cell comprises a first conductor, a steering element, a carbonnanotube fabric, and a second conductor, wherein the steering elementand the carbon nanotube fabric are arranged electrically in seriesbetween the first conductor and the second conductor, and wherein theentire carbon nanotube memory cell is formed above a substrate, thecarbon nanotube fabric having a first resistivity, the methodcomprising: applying a first electrical set pulse between the firstconductor and the second conductor, wherein, after application of thefirst electrical set pulse, the carbon nanotube fabric has a secondresistivity, the second resistivity less than the first resistivity. 2.The method of claim 1, wherein the steering element comprises a diode.3. The method of claim 2, wherein the diode comprises a junction diode.4. The method of claim 2, wherein the diode comprises a verticallyoriented p i-n diode.
 5. The method of claim 4, wherein the firstconductor is above the substrate, the second conductor is above thefirst conductor, and the diode and the carbon nanotube fabric arevertically disposed between the first conductor and the secondconductor.
 6. The method of claim 2, wherein the memory cell furthercomprises a silicide layer in contact with the diode.
 7. The method ofclaim 6, wherein the silicide layer comprises titanium silicide orcobalt silicide.
 8. The method of claim 1, wherein the carbon nanotubefabric is disposed between and is in contact with a top electrode and abottom electrode, the top electrode immediately above the carbonnanotube fabric and the bottom electrode immediately below the carbonnanotube fabric.
 9. The method of claim 1, wherein the substratecomprises monocrystalline silicon.
 10. A method comprising: providing amemory cell comprising a first conductor, a second conductor and asteering element and a carbon nanotube material arranged electrically inseries between the first conductor and the second conductor; andapplying a first electrical pulse between the first conductor and thesecond conductor to change a resistivity of the carbon nanotubematerial.
 11. The method of claim 10, wherein a data state of the carbonnanotube memory cell is stored in the resistivity of the carbon nanotubematerial.
 12. The method of claim 10, further comprising applying a readvoltage between the first conductor and the second conductor to sense afirst data state of the memory cell.
 13. The method of claim 10, whereinapplication of the first electrical pulse changes the resistivity of thecarbon nanotube material from a first resistivity to a secondresistivity.
 14. The method of claim 13, wherein the second resistivityis less than the first resistivity.
 15. The method of claim 13, whereinthe first resistivity is less than the second resistivity.
 16. Themethod of claim 13, further comprising: applying a second electricalpulse across the steering element and the carbon nanotube material,wherein after application of the second electrical pulse, the carbonnanotube material has a third resistivity, the third resistivity beinggreater than the second resistivity.
 17. The method of claim 16, furthercomprising: after applying the first electrical pulse, and before thestep of applying the second electrical pulse applying a read voltagebetween the first conductor and the second conductor, to sense a firstdata state of the memory cell; and after applying the second electricalpulse, applying a read voltage between the first conductor and thesecond conductor, to sense a second data state of the memory cell,wherein the first data state and the second data state are not the same.